Wed, 17. September 2014
Meade LX-90 Telescope Equatorial Mount - 2
After fitting the wedge to my LX-90 I have been outside with it three times and although I have not yet succeeded in accurately polar aligning it, I have progressed with it.
The five steps of alignment which I identified in the previous post remain as the principle requirements and I am still aiming to improve each technique. I can also add a sixth step, Drift Aligning, which I have not yet attempted.
So, to recap, the six steps to successful polar alignment of the LX-90 are:
Step 1. Setting up the wedge on the tripod before sunset and aligning it prior to attaching the telescope.
Step 2. Attaching the telescope base to the mount and rotating the tube manually to "Polar Home".
Step 3. Fine adjusting the mount alignment after dark by locating Sigma Octans in the finderscope.
Step 4. Aligning to Sigma Octans.
Step 5. Aligning the Autostar.
Step 6. Drift aligning.
What I learnt after three nights outside testing the first five of these techniques is as follows:
Steps 1 & 3 seem reasonably straightfoward and I think I can now cope with the minor issues which arise:
- The first two nights out I failed to recognise Sigma Octans and its associated asterism of fifth magnitude stars. This was mainly due to the difficulty in making fine azimth adjustments. Rotating the metal wedge on the metal tripod top was not easy. I am not sure of the best lubricant to use but on the third night I added a few drops of household oil before assembly and it permitted a smooth adjustment. Sigma was located after sitting directly behind the telescope (after dark) and realising that I had under-compensated for the magnetic deviation between Magnetic South and True South.
- I was not using the built in Meade compass - it seems useless. At home I used two compass apps on my Ipad which are designed to detect not only Magnetic South but also True South. Unfortunately when out in the field, the apps did not work without an internet signal and I ended up using a less accurate hand-held compass.
- The theory that, if the mount is flat and the latitude is correctly set, then the telescope mount must be pointing vertically upwards at the correct angle, proved correct. The weak link is horizontal magnetic deviation - and the lubricant made this innaccuracy easier to manage.
- A minor irritant was that the four star asterism does not entirely fit into the finderscope field of view.
LX-90 Wedge: Azimuth Control Adjuster
Step 2 seems to be a major weak link:
- I need to find an accurate way of ensuring the tube aligns with the forks.
- It is no good if the tube is at an angle to the forks - and that is surely the main reason for the inaccurate object alignments I noticed after completing the polar setup.
- I used a clinometer app on my Ipad to check that the angle of the two was identical - and they were certainly less than one degree apart.
- If these two are not aligned, then I suspect that attempts to correct any errors by drift aligning will not succeed.
LX-90 almost (but not quite) in the Polar Home position.
- Once Sigma Octans is in the field of view, it is necessary to continue to correct using the manual adjusters and the finderscope.
- Before this, the finderscope needs to be accurately aligned to the telescope.
Step 5 Aligning the Autostar:
- The "One Star" alignment procedure & messages seem to be ambiguous.
- At the end, there is no message except "successful".
- After doing this I searched for Saturn but the scope was about 5 degrees lower in RA, although Dec was close.
- I also twice tried the "Easy" alignment procedure, which was more simple.
- At the conclusion of the "Easy" method, a message is very briefly displayed.
- The second time, I noted the message, something like: "Pole is 192' 342'."
- This message is not self-explanatory but seems possibly to point towards a misalignment of 3 to 6 degrees!
- Following the second Easy Star alignment I found that Saturn did not appear to align in RA or Dec.
- I need to research this more online.
- I think I need to obtain greater accuracy in Steps 1-5 before going too far with this.
Further issues:
- Two of my three outings have been to dark sky sites and I need to spend more time at home learning all of these procedures, rather than wasting the effort to pack the car and drive out to the dark sky site.
- Finderscope alignment needs to be carried out early in the alignment procedure.
- Training the drives needs to be done again to reduce inaccuracies.
- Calibrating the sensors also needs to be carried out in Polar Mode.